Radio communication systems which transmit information between locations have recently been used in bar code-based information systems. Information systems that are based on bar codes typically include a base computer which serves as a storage location for information and as a source of commands which control the other components of the system. These information systems also include bar code readers which are used to collect bar code-based information and other components which cause the information and commands to be communicated between the base computer and the bar code readers. Typical applications for bar code-based information systems are in inventory management and retail sales.
In many bar code-based information systems, it is advantageous for the bar code readers to be mobile. In the past, this has been accomplished by providing the bar code readers with local memory and powering the readers by batteries. In operation, a mobile bar code reader of this type are used to collect information from bar codes, store this information in its memory and then physically connect the bar code reader to a hard-wired communication system which reads the bar code reader's memory and transmits the stored information to the base computer. This mode of operation is both time-consuming and inconvenient. It also prevents the immediate transmission of the information to the base computer.
One solution of this problem is to connect the mobile bar code readers to the base computer through a radio link. A particularly advantageous type of radio link is provided by a spread spectrum communication system, which allows reliable communication of information in an electromagnetically noisy environment without high radio frequency (RF) power levels. A spread spectrum communication system is highly resistant to interference from other such systems, and allows transmission into areas which are not along a line-of-sight. Spread spectrum communication systems have been described by R. A. Scholtz in "The Origins of Spread Spectrum Communications," IEEE Trans. Comm., vol . COM-30, May 1982, pp. 822-854. One very useful form of spread spectrum communication system is based on the asynchronous transmission of "packets" of information between a source and a destination. In a bar code information system, a bar code reader and the base computer can serve as either a source or a destination. If two-way communications are necessary between a bar code reader and the base computer, each unit will be both a source and a destination.
A packet radio communication system allows communication between each bar code reader and the base computer essentially whenever such communication is needed. After a communication link has been established between the bar code reader and the base computer, the information to be communicated is broken into small portions. These relatively small portions of information are sent from the source to the destination in discrete messages. One advantage of breaking the information into discrete messages is that, if one of the discrete messages is not received correctly, the destination can request a retransmission of that message without having to request a retransmission of more.
The form of the discrete messages is a string of digital information, called a "frame." The information in the frame is arranged in a sequence of "fields." The contents of each field in a frame is established by a protocol, such as those prescribed in packet radio standards X.25, AX.25 and ALOHANET, for example. The X.25 standard is described in ISO document 4335--"Data Communication--High level data link control procedures. Consolidation of elements of procedures," second edition 1984. ALOHANET is described by Norman Abramson, in "Development of the ALOHANET," IEEE Trans. on Info. Theory, Vol. IT-31, No. 2, March 1985.
Low power communication systems such as spread spectrum packet radio systems have only a limited range. Typically transmitters with output powers of one Watt have a range no greater than 500 feet. Accordingly, it is possible for a mobile bar code reader to move beyond a range where the base computer and the mobile bar code reader can communicate reliably. In situations where this occurs, it has been found that one or more repeaters can be placed between the bar code reader and the base computer. These repeaters receive the information transmitted by the source and retransmit the information to the destination. In situations where longer range packet radio communications are desirable, one or more repeaters can form a transmission path between the source and destination. One example of such a situation is where a mobile bar code reader is used in a very large warehouse for inventory control purposes.
If a packet radio communication system uses a repeater, it is possible for a message transmitted by the repeater to be received by both the source and the destination. It is therefore necessary to provide some means for the packet to be identified. In the protocols mentioned above, packet identification includes addresses for the source and destination. Therefore, it is possible for a source to ignore a message which is repeated back to the source by recognizing that it was the source for the repeated message.
The addition of repeaters to a packet radio communication system introduces further complications. If more than one repeater can receive a message transmitted by a source, more than one repeater will repeat the message. If the destination is within range of the multiple repeaters, it must be capable of determining that it has received more than one copy of the same message and ignoring all but one copy. Further, it is possible for a message which has already been received by one repeater to receive that same message subsequently from another repeater. It is therefore advantageous to provide a protocol which will reduce the unnecessary repetition of messages in a packet radio communication system.
There is a finite possibility that messages can be lost in a packet radio communication system. Packet radio communications generally provide for confirmation of receipt of messages. In many systems, if a message is received incorrectly, the message is regarded as lost and retransmitted. This requires that the retransmitted message must be properly identified. It is therefore desirable to provide a protocol which will allow a lost message to be identified and a corresponding retransmitted message to be correctly placed in proper sequence in the information transmitted between the source and destination.